There are many disease states that cause bone defects in the spinal column. For instance, osteoporosis and other metabolic bone conditions weaken the bone structure and predispose the bone to fracture. If not treated, certain fractures and bone defects of the vertebral body may produce intolerable pain, and may lead to the development of deformity and severe medical complications.
Bone weakening may also result from benign or malignant lesions of the spinal column. Tumors often compromise the structural integrity of the bone and thus require surgical stabilization and repair of defects with biocompatible materials such as bone grafts or cements. Bone tumors of the spine are relatively common, and many cause vertebral compression fracture.
More than 700,000 osteoporotic compression fractures of the vertebrae occur each year in the United States—primarily in the elderly female population. Until recently, treatment of such fractures was limited to conservative, non-operative therapies such as bed rest, bracing, and medications.
One surgical technique for treating vertebral compression fracture can include injecting or filling the fracture bone or bone defect with biocompatible bone cement. A relatively new procedure known as “vertebroplasty” was developed in the mid 1980's to address the inadequacy of conservative treatment for vertebral body fracture. This procedure involves injecting radio-opaque bone cement directly into a fracture void, through a minimally invasive cannula or needle, under fluoroscopic control. The cement is pressurized by a syringe or similar plunger mechanism, thus causing the cement to fill the void and penetrate the interstices of a broken trabecular bone. Once cured, the cement stabilizes the fracture and eliminates or reduces pain. Bone cements are generally formulations of non-resorbable biocompatible polymers such as PMMA (polymethylmethacrylate), or resorbable calcium phosphate cements which allow for the gradual replacement of the cement with living bone. Both types of bone cements have been used successfully in the treatment of bone defects secondary to compression fractures of the vertebral body.
One clinical issue associated with vertebroplasty is containment of the cement within the margins of the defect. For instance, an osteoporotic compression fracture usually compromises portions of the cortical bone creating pathways to cement leakage. Thus, there is a risk of cement flowing beyond the confines of the bone into the body cavity. Cement leakage into the spinal canal, for instance, can have grave consequences to the patient.
Yet another significant risk associated with vertebroplasty is the injection of cement directly into the venous system, since the veins within the vertebral body are larger than the tip of the needle used to inject the cement. A combination of injection pressure and inherent vascular pressure may cause unintended uptake of cement into the pulmonary vessel system, with potentially disastrous consequences including embolism to the lungs.
One technique which has gained popularity in recent years is a modified vertebroplasty technique in which a “balloon tamp” is inserted into the vertebral body via a cannula approach to expand or distract the fractured bone and create a void within the cancellous structure. Balloon tamps are inflated using pressurized fluid such as saline solution. The inflation of a balloon membrane produces a radial force on the bone and forms a cavity in the bone. When deflated and removed, the membrane leaves a cavity that is subsequently filled with bone cement. The formation of a cavity within the bone allows for the injection of more viscous cement material which may be relatively less prone to leakage.
In certain instances, such as the treatment of acute or mobile fractures, the balloon is also effective at “reducing” the fracture and restoring anatomic shape to a fractured body. In particular, balloon dilatation in bone is maximally effective if the balloon device is targeted inferior to, or below, the fracture plane. In this instance, the balloon dilatation may distract, or lift, a fracture bone fragment, such as the vertebral body endplate.
In other instances, such as chronic or partially healed fractures, balloons are less effective at “reducing” the fracture because radial forces are insufficient. Often the bone in an incompletely healing fracture is too dense and strong, and requires more aggressive cutting treatment, such as a drill or reamer tool to create a sufficient cavity. In these more challenging cases, the ability to inject bone cement into a cavity created by a balloon or a reamer in the vicinity of the fracture is typically sufficient to stabilize the bone and relieve pain, even in the absence of fracture reduction.
One limitation to the use of such methods has been the difficulty in targeting the location at which the cavity should be created. Known techniques require access to the vertebral body using straight cutting and reaming tools which are only able to access a limited region of the vertebral body being treated, generally only within one side of the vertebral body. A cavity created using these techniques can only treat one side of a vertebral body being targeted, resulting in an uneven distribution of bone cement that cannot completely stabilize the vertebral body. As a result, multiple entry points on different sides of the vertebral body are generally required in order to provide a symmetrical distribution of bone cement around a central axis of the vertebral body. These multiple entry points significantly increase the time necessary for the procedure, the portion of the body being treated, and the amount of bone cement being injected, and as such can significantly increase the risks associated with treatment of a patient.